Mixing human and mouse cells
According to the New York Times, scientists want to create a new kind of mouse-human hybrid by inserting human stem cells into a mouse embryo. The resulting creature would have both human and mouse cells.This is different than genetic engineering, because all the cells in the creature would be either fully human or fully mouse. (In genetic engineering, all cells have the same genetic makeup, which may be a mixture of creatures.)
From the article: "Dr. Weissman of Stanford, who was not at the meeting, said the experiment could help scientists follow the behavior of human cells with genetic diseases. Studying how the diseased human cells develop in a mouse could offer treatment insights."